1. Field of the Invention
The present invention relates to a Low Noise Amplifier (LNA), and more particularly, to a Low Noise Amplifier (LNA) having a variable gain mode.
2. Description of the Background Art
In wireless apparatus for receiving signal such as a mobile phone and a TV set, a first stage is commonly formed of an amplifier for amplifying a received signal.
In such an amplifier, when the strength of the received signal is very small, an amplification operation having a low noise and high gain characteristic is required. However, when the strength of the received signal is relatively large, an amplification operation having a linear characteristic is required.
Therefore, in the wireless apparatus, the amplifier has two or more amplification modes in accordance with an input signal level to select one among the amplification modes.
A low noise amplifier (LNA) capable of switching a low gain state and a high gain state is disclosed in U.S. Pat. No. 6,144,254.
FIG. 1 is a circuit diagram illustrating the LNA disclosed in the U.S. Pat. No. 6,144,254.
As illustrated in FIG. 1, the LNA comprises a common emitter type first NPN transistor BN1 that operates in the high gain state, a common base type second NPN transistor BN2 that operates in the low gain state, and a third NPN transistor BN3 and a resistor R1 for supplying a bias current to the second NPN transistor BN2.
That it, the collector of the first NPN transistor BN1 is connected to the output port Pout of the LNA, the base of the first NPN transistor BN1 is connected to the input port Pin of the LNA and a first bias input port Bias1, and the emitter of the first NPN transistor BN1 is grounded.
A resistor R1 is connected between the first bias input port Bias1 and the first NPN transistor BN1.
The collector of the second NPN transistor BN2 is connected to the output port Pout of the LNA, the base of the second NPN transistor BN2 is connected to a second bias input port Bias2, and the emitter of the second NPN transistor BN2 is connected to the input port Pin of the LNA and the collector of the third NPN transistor BN3.
The base of the third NPN transistor BN3 is connected to a third bias input port Bias3 and the emitter of the third NPN transistor BN3 is grounded.
Hereinafter, the operation of the conventional LNA will be described with reference to FIG. 1.
In the high gain state, the first bias input port Bias1 is at a high level and the second and third bias input ports Bias2 and Bias3 are at a low level.
Therefore, in the high gain state, the common emitter type first NPN transistor BN1 is activated to perform an amplification operation of high gain. In this case, the second and third NPN transistors BN2 and BN3 are turned off.
In the low gain state, the second and third bias input ports Bias2 and Bias3 are at a high level and the first bias input port Bias1 is at a low level.
Therefore, in the low gain state, the common base type second and third NPN transistors BN2 and BN3 are activated to perform an amplification operation of low gain. In this case, the first NPN transistor BN1 is turned off.
The LNA illustrated in FIG. 1 selects the high gain state and the low gain state to perform the amplification operation of the high gain or the amplification operation of the low gain in accordance with the strength of the received signal.
However, in the LNA illustrated in FIG. 1, the input port of the common emitter type first NPN transistor BN1 used in the high gain state and the input port the common base type second NPN transistor BN2 used in the low gain state, that is, the base of the common emitter type first NPN transistor BN1 and the emitter of the common base type second NPN transistor BN2 are directly connected to each other so that circuits that operate in the respective gain states have influences on each other as loads.
That is, when the LNA operates in the high gain state, the capacitance of the emitter terminal of the common base type second NPN transistor BN2 used in the low gain state operates as the load of the high gain circuit to have bad influence on the performances such as the gain, matching, and noise characteristics of the high gain state and to thus deteriorate the performance of the LNA.
Also, when the LNA operates in the low gain state, the capacitance of the base terminal of the common emitter type first NPN transistor BN1 used in the high gain state deteriorates the performance of the low gain state like in the high gain state.
This is because the impedance levels of the input terminals of the two modes that are actually the same operate as loads to each other.